A typical assembled circuit board includes a printed circuit board and multiple circuit board components (e.g., integrated circuit (IC) packages, discrete components, connectors, etc.) which are solder mounted to the printed circuit board. Conventional solder mounting approaches include pin-in-hole (through-hole) soldering and surface mount technology (SMT) soldering.
There are different types of solder for mounting components to printed circuit boards. Although tin-lead (Sn—Pb) solders are the most common, manufacturers are moving away from lead-based (Pb-based) solders toward lead-free (Pb-free) solders due to environmental concerns and stricter government regulations. Pure tin (Sn) is an example of a Pb-free solder currently used by manufacturers.
Unfortunately, Pb-free finishes are susceptible to the spontaneous growth of “tin whiskers” or more generally “metallic whiskers” (i.e., metallic crystal structures). Such growths can cause electrical failures ranging from parametric deviations to catastrophic short circuits. Additionally, such growths may physically interfere with circuit board operation such as contaminate sensitive optical surfaces and hinder the movement of Micro-Electro Mechanical Systems (MEMS) devices.
Although metallic whiskers have been studied and reported for decades, the mechanism behind their growth is not well understood, and they remain a potential reliability hazard. Furthermore, the growing number of piece parts with pure tin finishes means there are more opportunities for metallic whiskers to grow and to produce failures.
One conventional approach to addressing the growth of metallic whiskers is referred to as “hot-solder dipping”. Hot-solder dipping involves dipping the leads of circuit components into molten Sn—Pb solder. Such replating of the leads reduces the amount of lead having a whisker-prone metallic plating composition and thus tends to reduce the incidence of metallic whisker growth.
Another conventional approach to addressing the growth of metallic whiskers is referred to as “conformal coating”. Conformal coating involves coating the entire assembled circuit board (e.g., a populated circuit board that has been built and initially tested) with an epoxy material. The epoxy material reduces metallic whisker formation as well as provides protection against moisture, dust, corrosion, etc.